1. Field
The present disclosure relates to a capacitive deionization device, and more particularly, to a capacitive deionization device that includes at least one electrolyte solution having ionic species contained therein, the types and/or total concentration of which differ from those of ionic species contained in influent water to the capacitive deionization device; and at least one electrolyte compensation device in fluid communication with the at least one electrolyte solution.
2. Description of the Related Art
A capacitive deionization (“CDI”) device is used to remove an ionic material from a medium, for example, hard water, typically by applying a voltage to a pair of electrodes having nano-sized pores in order to polarize the pair of electrodes, so that the ionic material is adsorbed onto a surface of at least one of the pair of electrodes. In the typical CDI device, when a low direct current (“DC”) voltage is applied to the pair of electrodes while the medium containing ions dissolved therein flows between the two electrodes, wherein one of the electrodes functions as a positive electrode and the other of the electrodes functions as a negative electrode, anions dissolved in the medium are adsorbed and concentrated in the positive electrode, and cations dissolved in the medium are adsorbed and concentrated in the negative electrode. When a current is supplied in a reverse direction, e.g., by electrically shorting the two electrodes, the concentrated ions are desorbed from the negative electrode and the positive electrode. Since the CDI device does not use a high potential difference, the energy efficiency thereof is high. Furthermore, the CDI device may also remove detrimental ions as well as hardness components when ions are adsorbed onto the electrodes, and does not use a chemical to regenerate the electrodes and thus the typical CDI device has a relatively low environmental impact.
However, in the general CDI devices, when a potential is applied to the electrodes, a large number of ions, i.e., co-ions, present in pores of the electrodes with the same polarity as the corresponding electrodes are expulsed into effluent water. As such, it is difficult to control all ions to be moved towards the corresponding electrode. For this reason, typical CDI devices have a relatively low ion removal efficiency compared to the amount of applied charges.
In order to address these drawbacks of such general CDI devices, Andelman et al. (U.S. Pat. No. 6,709,560) introduce a charge-barrier CDI device including a charge barrier such as an ion exchange membrane to improve the ion removal efficiency of the CDI device.
The charge-barrier CDI device is advantageous, as compared to general CDI devices, when it is used to treat water, such as seawater, containing a high concentration of ions, wherein the prevention of co-ion expulsion is important. However, when the charge-barrier CDI device is used to treat hard water including a hardness component of 300 ppm or less by weight, the concentration of ions in pores of the electrodes is relatively low and the ion transfer rate in the pores is also low. Thus, the capacitances of electrode materials may not be fully utilized during charging/discharging.
In addition, such general CDI devices and the charge-barrier CDI device exhibit a lower ion removal efficiency when influent water to be treated contains ions (hereinafter, “detrimental ions”) unsuitable for exhibiting capacitance with the electrode material.